1. Field of the Invention
The present invention relates to a phase-shifted distributed feedback (DFB) type semiconductor laser diode (DFB-LD) and its manufacturing method.
2. Description of the Related Art
DFB-LDs have been used as light sources in high-speed, long-distance and large-capacity optical fiber communications. In directly-modulated DFB-LDs, whose output amplitude is modulated by a pump circuit, the carrier density within an active layer and the equivalent refractive index fluctuate, which induces a spectrum spread called a dynamic wavelength shift or a wavelength chirp.
In order to suppress the wavelength chirp, a prior art phase-shifted DFB-LD has been suggested (see: JP-A-2000-077774 and JP-A-2000-277851). That is, a xcex/n phase shift (n greater than 4, preferably, n=5xcx9c8) where xcex is an oscillation wavelength is located at a diffraction grating of a waveguide.
Generally, in a DFB-LD, the fluctuation of a Bragg deviation amount xcex94xcex2 is opposite in phase to the fluctuation of the optical output. In this case, note that a Bragg deviation amount xcex94xcex2 is defined by
xcex94xcex2=2neqxcfx80(1/xcexxe2x88x921/xcexB)
where neq is an equivalent refractive index;
xcex is an oscillation wavelength; and
xcexB is a Bragg wavelength determined by the period of the diffraction grating, i.e., twice the period of the diffraction grating.
Also, assume that the phase shift value is less than xcex/4, for example, xcex/5xcx9cxcex/8. In this case, the larger the Bragg deviation xcex94xcex2, the smaller the mirror loss xcex1m. Note that the Bragg deviation xcex94xcex2 and the mirror loss xcex1m determine an oscillation mode. Further, the smaller the mirror loss xcex1m, the larger the optical output. Therefore, when the optical output is increased by the external reflection return light, the Bragg deviation xcex94xcex2 is decreased so that the mirror loss xcex1m is increased, thus decreasing the optical output. Contrary to this, when the optical output is decreased by the external reflection return light, the Bragg deviation xcex94xcex2 is increased so that the mirror loss xcex1m is decreased, thus increasing the optical output. Therefore, a negative feedback control by the external reflection return light is performed upon the optical output, so that the fluctuation of the optical output can be suppressed, which also suppresses the wavelength chirp.
Note that JP-A-2000-277851 provides a xcex/5 to xcex/8 phase-shifted DFB-LD including a multiple quantum well (MQW) active layer formed by a tensile-strained well layer, thus realizing the above-mentioned negative feedback control.
In the above-described prior art phase-shifted DFB-LD, however, since the wavelength chirping characteristics and the transmission characteristics strongly depend on parameters of the DFB-LD, the wavelength chirping and transmission characteristics cannot be improved. Note that the wavelength chirping characteristics dominates the transmission characteristics.
It is an object of the present invention to provide a phase-shifted DFB-LD capable of improving the wavelength chirping and transmission characteristics.
Another object is to provide a method for manufacturing such a DFB-LD.
According to the present invention in a DFB-LD including a semiconductor substrate, an optical guide layer formed on the semiconductor substrate, a diffraction grating having a phase shift region being formed between the semiconductor substrate and the optical guide layer, and an active layer formed on the optical guide layer,
xcexaL+Axc2x7xcex94xcexxe2x89xa7B
where xcexa is a coupling coefficient of the diffraction grating, L is a cavity length of the diode, xcex94xcex is a detuning amount denoted by xcex94=xcexgxe2x88x92xcex where xcexg is a gain peak wavelength of the diode and xcex is an oscillation wavelength of the diode, A is a constant from 0.04 nmxe2x88x921 to 0.06 nmxe2x88x921, and B is a constant from 3.0 to 5.0.
Also, in a method for manufacturing a phase-shifted DFB-LD, a plurality of samples of the phase-shifted DFB-LD having different normalized coupling coefficients xcexaL and different detuning amounts xcex94xcex are formed. Next, power penalties of the samples connected to an optical fiber are measured. Next, values of the normalized coupling coefficients xcexaL and the detuning amounts xcex94xcex of the samples with the power penalties are plotted in a graph. Next, xcexaL+Axc2x7xcex94xcex=B is determined where A and B are constants in order to divide the samples into first and seconds areas in the graph, so that most of the samples belonging to the first area have power penalties smaller than a definite value and most of the samples belonging to the second area have power penalties not smaller than the definite value. Finally, a new phase-shifted DFB-LD having a normalized coupling coefficient xcexaL and a detuning amount xcex94xcex satisfying xcexaL+Axc2x7xcex94xcexxe2x89xa7B is formed.